Method for removing acetylene from ethylene



Oct. 17, 1961 METHOD J. E. COTTLE FOR REMOVING ACETYLENE FROMETHYLENE PRODUCT C 5 5 PURIF. 9 4 IO ll (flag) 'xH O, Q I DMF 4 J r' DMF I -2 a N5 -e II I! E 2 n. O w E CD U) SECONDARY STRIPPER ETI'HYLENE KY Mm INVENTOR. J.E. COTTLE A TTORN VS ethylene overhead product from the absorber. other. of its aspects, the invention relates to the injection United rates Patent 3,004,629 METHOD FOR REMOVING ACETYL'ENE FROM ETHYLENE John E. 'Cottle, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Filed Sept. 25, 1959, Ser. No. 842,418 4 Claims. (Cl. 183115) This invention relates to an improved method for the removal of acetylene from ethylene. In one of its aspects, the invention relates to a method for the recovery of acetylene from a mixture of gases containing ethylene and acetylene, employing dimethylformamide as an absorbing agent, using water to decrease the solubility of ethylene in the dimethylfoimamide yet recovering a dry In anof water into an absorber in which dimethylformamide is employed to absorb acetylene from a mixture of gases containing it and ethylene to obtain intercooling in the absorption zone.

The separation of acetylene from a mixture of gases containing it and ethylene is well known. It is known to use dimethylformamide as a solvent for the separation.

It has now occurred to me that the introduction of .water to an acetylene absorption column either separately or together with at least a portion of the dimethylformamide being introduced to the column will favorably affeet the separation of the minor amount of acetylene which is contained in the ethylene gas being treated. The

,use of water will alter the characteristics of the solvent to somewhat reduce the solubility of both the ethylene and the acetylene at the point of introduction of the water resulting, however, in an increased selectivity for acetylene as against ethylene, thus increasing the amount of substanject of this invention to provide an improvement'in the separation of acetylene from ethylene employing dimethylformamide as a solvent. It is a still further object of the invention to provide animproved method for the separation of acetylene from ethylene employing dimethylformamide as a solvent to yield increased recovery of acetylene free, substantially dry ethylene. Another object of the invention is the production of relatively high purity C H Other aspects, objects, and the several advantages of the invention are apparent from a study of the disclosure, the drawing, and the appended claims.

' According to the present invention, there is introduced into the bottom portion of an acetylene from ethylene absorber, employing dimethylformamide as a downwardly flowing acetylene removal solvent, water in a quantity sufficient to decrease the ethylene solubility in the dimethylformamide solvent in the bottom portion of the absorber but in a quantity insuflicient to prevent absorpsufficient to significantly affect the overall absorption of ice C H Usually 1-25 mol percent water will be introduced into the column based on the total solvent feed to the column. 7

Referring now to the drawing, dirnethylformamide introduced at 1 into tower 2 is contacted therein with rising ethylene containing gas introduced into tower 2 by way of pipe 3. The dimethylformamide is essentially anhydrous. The tower is washed down at its upper portion by liquid ethylene introduced by pipe 4. Purified ethylene is taken off overhead by pipe 5. According to the invention the yield of ethylene coming off at pipe 5 is increased by introducing water to the lower portion of column 2 by pipe 6. The water is now preferred to be introduced as a dilute dimethylformamide aqueous solution. Introduction of water as a dilute aqueous dimethylformamide solution prevents its freezing upon entering the low temperature absorber. Solvent enriched with acetylene is withdrawn by pipe 7 and passed to stripper 8 from the top of which a gaseous stream consisting essentially of acetylene is removed by way of pipe 9, cooler 10, and knockout drum 11 to acetylene storage by way of pipe 12. If desired, stream 7 may be subjected to one or more flashing steps, not shown, prior to its introduction into stripper 8. C H thus flashed is compressed and returned to the absorber. This will further reduce the amount of C H lost via line 12. A dilute water solution of dimethylformamide is removed from knockout drum L1 and passed by way of pipe 6 to tower 2 for use as described herein. Bottoms from stripper 8 are passed by way of pipe 13' to additional solvent recovery vessel 14 from the bottom of which dimethylformamide is passed by way of pipe 1 and cooler 16 to tower 2. Some additional acetylene, water vapor, and dimethylformamide vapor pass by way of pipe 15 to pipe 9 for recovery together with the streamalready in pipe 9 coming from stripper 8.

Introduction of the water at an intermediate point, or points, according to the invention, will result in evolution of ethylene from the liquid phase, causing a reduction in temperature. Thus, according to the invention, intercooling is obtained in the intermediate portion of the tower. V

The dry dimethylformamide introduced by way of pipe 1 can be presaturated with liquid ethylene as set forth and described in copending application Serial Number 624,-

465, filed November 26, 1956 by Robert A. Koble.

It now appears, based on data which have been obtained, that acetylene concentration in the dimethylformamide stripper overhead C hydrocarbon stream can be increased to better than percent according to the present invention. This will, of course, considerably reduce ethylene loss in the acetylene flowing from this stripper and this ethylene will be recovered from the acetylene absorber. V

Further, water vapor at the top of the stripper, resulting from the introduction of water to the absorber, tends to eliminate completely the explosion hazard ahead of the now conventional condenser. Since the dimethylformamide will tend to stay with the water, which is present according to the invention, a refrigerated condenser to knock out the dimethyl-fornramide, conventionally used, can be considerably reduced in capacity or completely eliminated, depending upon the extent of recovery desired.

Since intercooling is obtained due to the evolution of 3 ethylene in the-lower port-ion of the absorber, a reduction in the dimethylformamide solvent circulation can desirably be practiced in some instances.

It is important to note, in the operation of the method of the invention, the concept that the water vapor which 5 tends to rise in the absorber column is absorbed by the cold, dry dimethylformamide solvent passing downwardly through the column thus resulting in a dry ethylene 0&- gas stream.

emotions o iorera i az n th a s e n he em- Eb9 imQI1 e sar be a e pn oxi aatel s llow The dimethylformamide solvent and liquid ethylene .contact the-r sing gases a ga t nps iiltur at 0 and a .a p sure of -,apP 9x mat. lv; .00 pound pe squa nch abso- ,lute pressure. The feed stream .of gas .ir m whi acetylene ,is .to :be removed will have the following com position.

Molsper day Acetylene 150 l-Ethylene 10,000 Ethane 90 Total 10,240

The bottorn tray of the absorber willbe at a'tempera- :ture of approximately 20F. .Anhydrous dimethyl formamide is introduced to .the tower at an approximate rate 1 of 3,310mols .periday. Aportionof the purified ethylene takenoverhead from the absorber .is liquefied and returned to the topofthe column at a rate. of approximately 1,950 mols per day. This quantityof .ethyleneis just sufficient to saturate the liquid dimethylformamide intro- *duced to the absorber.

The amount of water introduced, as stated, can be varied-to be sufiicient to accomplish some-of the benefits of the invention but insufiicient to prevent obtaining'a dry, purified ethylene overhead "gas from the absorber. In the embodiment :described, there will be introduced j'660I1'1Q1S PfiI' day of water, containing approximately 10 mols per day of dimethylform-amigle.-

One skilled inthe art'in possession of this disclosure, having studied the same will beableto readily design and construct and successfullyoperate an acetylenetrum ethylene absorption system according to the invention.

With respect to further characteristics and details which maybe helpful, gthe information of Serial Number 624,- 465 above mentioned, is incorporated herewithby reference.

Solubility data for acetylene in the aqueous dimethyl- 'formamidein-the bottom portion ofthe column as contained in a feed stream treated by the system age as follows;

-.C H solubility in 80420 .volume Y percent :DMETH O at CzHaAD tfiI Qfi H Ahsolut p s ure 0 10 s ;,57.I- 0. 1 90.5 0.0019 Y r 93,0 1.0.0023 109.0 .00029 1.250

:Q H solubility-same conditions.

1'C 'H .molz fraction: :Pressure -p.s,i.a.

v t .n -me e e ene a s th ti v at oaz h weight ratio of water to dimethylformamide solvent-presentjn the tower ,at the point, ofintroductionofthejnater will be 16 to 1.

The following are quantities .of specific flows in a .unit operated according to the invention and are given by way of an example in which units are given in mols per day.

Stream 3: Stream 5:

C 5 150 C 90 C 10,000 C 11,895 C 90 C 5 Trace DMF Nil 10 Stream 4: F Pam 1:

Stream 7: Stream .6: 2 .50

C 55 DMF 10 DMF 3,320 H 0 660 Stream 12:

C 55' 25. With respect to the selectivity of the invention, i.e.,

the solubility of acetylene and ethylene, there is inform ation showing .that in wet dimethyltormamide the activity .coefiicient for C H is about 28, that is ten times higher than .in dry dimethylformamide. In view of the higher activity coefficient there would be less ethylene in the bottoms when operating the present invention. IIhus,}whereas in stream 7 the ratio of C H :C H is 257:1, ,in the same system with dry dimethylformamide .it would be but 0.2:1. ;In an absorption column containing, for example, 40 {trays and having a diameter of 3 /2 feet and a gas thr oughputqf 10,240 .mols per day, the aqueous ,DMF may-he introduced into the column at any point below the 8th tray .-fr om the top. of the absorber.

.Reasgnable variation and modification are PQ Sible .within the scope the toregoing disclosure, ,thedraw- .ing, and thegppended claims to the invention the essence ,of ,which is (that ,in ,a dimethylformamide solvent removal. o-f acetyle e and eth en Water is p y i .the lower .portion of'the absorber thus increasing the yield of y, pur fie eth n an Obtaining he ad- .vantagesas .evident from :a s'gudy of v disclosure.

'Lelaimz 21. A method ior the purification of an ethylene stream also containing acetylene which comprises contacting said .stream with a .streamof dry dimethylformamide, =flowing the streams countercurrently to each other, introd=ucing water to the .thus contacted streams after substantial contactof them .has beenobtained so as to release ethylene from the solvent; Phase, ,thus removing -from -the' contacting operation .a solvent enrichedwith acetylene .and an essentially vdry ethylene gas substantially -free -from acetylene, .the quantity of water introduced being suificientto reduce the .solubility of ethylene in the dimethyl-formarnide solvent but insuflicient to cause water vapor to .be present in the purified ethylene stream.

2. A method for {the purification of ethylene -,.which comprises introducing an ethylene vand acetylene conltai "n g gas to the'bottom of an absorber .zone and passing the same upwardly countercurrent to adownward- =ly moving stream of essentially dry dimethylt'ormamide,

introducing wagertothe absorber' zone at a-pointinterimediate [the introduction of the ethylene containing ,stream and thedimethylformamide entry to the absorber zone, the; mount of the water being sufiicient to reduce t he soln 'l ty ofiethylene in the dimethylformamide but ,ins,uifi cientgto pe rn iit wateryapor to be liberated in the 7 absorber zone in a quantity in excess of that whic ca be captured by the downwardly flowing dimethylformamide, recovering purified ethylene from the top of the 1 ratio of water to dirnethylformamide in said aqueous solution is in the range of from 1:1 to 24:1.

References Cited in the file of this patent UNITED STATES PATENTS 2,371,908 Morris et a1. Mar. 20, 1945 2,809,710 Hachmuth Oct. 15, 1957 2,871,979 Scofieid Feb. 3, 1959 2,907,409 Koble Oct. 6, 1959 

1. A METHOD FOR THE PURIFICATION OF AN ETHYLENE STREAM ALSO CONTAINING ACETYLENE WHICH COMPRISES CONTACTING SAID STREAM WITH A STREAM OF DRY DIMETHYLFORMAMIDE, FLOWING THE STREAMS COUNTERCURRENTLY TO EACH OTHER, INTRODUCING WATER TO THE THUS CONTACTED STREAMS AFTER SUBSTANTIAL CONTACT OF THEM HAS BEEN OBTAINED SO AS TO RELEAST ETHYLENE FROM THE SOLVENT PHASE, THUS REMOVING FROM THE CONTACTING OPERATION A SOLVENT ENRICHED WITH ACETYLENE AND AN ESSENTIALLY DRY EHTYLENE GAS SUBSTANTIALLY FREE FROM ACETYLENE, THE QUANTITY OF WATER INTRODUCED BEING SUFFICIENT TO REDUCE THE SOLUBILITY OF ETHYLENE IN THE DIMETHYLFORMAMIDE SOLVENT BUT INSUFFICIENT TO CAUSE WATER VAPOR TO BE PRESENT IN THE PURIFIED ETHYLENE STREAM. 